Mesozoic and Ccenozoio Geology and Palceontology. 247 



has held its place amid changes of great importance. It was during 

 the latter portion of the Tertiary age that much of the volcanic ac- 

 tivity took place which was so general over this portion of the country, 

 though probabh^ only the closing stages of the lava flows are repre- 

 sented by the eruptive deposits of the Pliocene epoch. A section on 

 the present lake shore, between Bluff Point and Steam Point, in de- 

 scending ordei", is as follows: 



1. Grass-covered soil passing graduall}' to loose sand, 2 feet. 



2. Various sand, gravel, and spring deposits with scattered irony 

 concretions, 6 feet. 



3. White and dark lake sand, \exy thinly laminated with beach 

 structure, and occasional irony la3'ers, 5 feet. 



4. About 15 feet of thinly laminated, blue-black clay, locally con- 

 torted and beautifully cut by a small rill, emanating as a spring from 

 one of the irony layers in No. 3. The water is slightly chalybeate. 



Other sections show the same general features with more or less vari- 

 ation. They represent the upper portion of the Pliocene series, de- 

 posited toward the close of volcanic activity, hence the occasional beds 

 of volcanic ejectamenta which were poured out into the lake, are main- 

 ly composed of volcanic sand and the finer textured conglomerates, as 

 may well be seen near Steamboat springs. As we descend the valley 

 of the Yellowstone river, we find the lower members of the group well 

 exposed, and the beds of unmodified non-molten material becoming- 

 more common, with increasing proportions of the molten or lava series, 

 untilthe latter are almost universal, and doubtless represent an earlier 

 period, though frequently largely concealed by the subsequent spring- 

 deposits. Near the close of the Pliocene epoch, the internal fires had 

 so far died out that the igneous ejections were of fitful occurrence, and 

 geysers, solfataras, fumaroles, etc., abounded to an almost incredible 

 extent, giving rise to enormous deposits of siliceous and calcareous 

 material, which has continued to be deposited with decreasing vigor 

 until the present day. 



Prof. G. K. Gilbert'* found a section of Tertiarj'^ on the east face of 

 Sam Pitch Plateau, at Wales, Utah, 1,292 feet in thickness, another 

 near the head of the main Sevier river, in Utah, 560 feet, and another 

 on the north fork of Virgin river, between Mountain Lakelet and 

 Rockville, in Southern Utah, estimated at 3,000 feet. 



Prof. E. D. Copef described the Puerco marls as in all probability 



* Geo. Sur. W. 100th Meridian, vol. iii. 



t Ann. Rep. Explr and Sur., W. 100th Meridian App. L. L. 



